Piezoelectric performance enhancement of ZnO flexible nanogenerator by a CuO–ZnO p–n junction formation

The piezoelectric potential screening by large excess electrons in nominally undoped ZnO has limited the energy conversion efficiency of the ZnO nanogenerators (NGs). In this study, we report a simple and effective approach to enhance the piezoelectric output performance of the ZnO NGs by forming a CuO–ZnO heterostructure. By depositing a ZnO thin film on the pre-deposited CuO thin film, which forms a p–n junction, excess electrons in ZnO can be effectively reduced. Thus, the piezoelectric potential generated in ZnO by an applied force can be less affected. Using this approach, we obtained an output voltage up to ∼7.5 V and a maximum current of 4.5 μA cm−2 measured under the forward connection, which is a 7-fold higher output voltage and an approximately one order of magnitude higher current density by comparison to the ZnO NGs without a CuO layer. Our results clearly demonstrate the effectiveness of a CuO–ZnO heterostructure for realizing high performance flexible energy harvesting devices.